Communication systems that employ linear polarization transmit electromagnetic waves wherein the electric field is confined to a particular plane along the direction of wave propagation. Vertical polarization and horizontal polarization are two prevalent types of linear polarization that are employed by many communication systems. A vertically polarized antenna generates an electric field that is perpendicular to the Earth's surface and a magnetic field perpendicular to the electric field and parallel to the earth's surface. In contrast, a horizontally polarized antenna generates an electric field that is parallel to the Earth's surface and a magnetic field perpendicular to the electric field and perpendicular to the earth's surface.
In communication systems that use circular polarization, the electromagnetic signal or radio frequency (RF) signal is transmitted such that the tip of the electric field vector describes a circle in any plane normal to the direction of signal propagation. However, in communication systems that employ elliptical polarization, the RF signal is polarized such that the tip of the electric field vector describes an ellipse in any plane normal to the direction of signal propagation. Circular polarization represents a limited type or subset of the more general elliptical polarization, wherein the tip of the electrical field vector describes a circle in a plane normal to the direction of signal propagation. Circular polarization may be regarded as left-handed circular polarization or right-handed circular polarization depending on the direction that the electric field vector rotates as the signal wave propagates past a particular point.
As compared with communication systems that employ linear polarization for radio frequency signal transmission and reception, communication systems that employ elliptical polarization provide several benefits. For example, systems employing elliptical polarization may suppress some multipath reflection components. Elliptical polarization systems may also penetrate through scattering media better than systems using other polarization types. In radar applications, systems that employ elliptical polarization may exhibit better range and resolution for polarization dependent targets.
The U.S. Pat. No. 5,764,696 to Barnes, et al. contains examples of “chiral” polarization used in ultra-wideband (UWB) transmissions. Chiral polarization requires increased transmission time for the signal as compared with linear polarization. Thus, chiral polarization unfortunately negatively impacts the traffic capacity of communication systems that employ this technology. Barnes shows a type of chiral polarization wherein one antenna emits a signal and a second antenna emits the same signal delayed in time. The two antennas are perpendicular to one another and are thus in a space quadrature relationship. In such chiral polarization, one antenna is effectively supplied with a time delayed copy of the signal supplied to the other space-quadrature antenna. While the chiral radiated polarization vector does appear to an observer to rotate in space, the polarization is not truly circular and does not have all the known benefits of true circular polarization. Because of the needed delay time, the transmission time with chiral polarization is equal to the pulse duration plus the time delay between the pulse sent in one linear polarization and the corresponding delayed pulse sent in the second linear polarization. In chiral polarization, the signal may be delayed by an amount of time greater than a pulse width. Hence, chiral polarization may unfortunately consume twice the channel time of a linear polarization.
Circular polarization may be helpful in suppressing multipath propagation components that involve one or any odd number of reflections because an odd number of reflections inverts the circular polarization sense. Thus, upon one or an odd number of reflections, left-handed circular polarization inverts to right-handed circular and vice-versa. In a typical in-room scenario for short range high data rate UWB communications, a radar scenario or an imaging scenario, a single reflection may be only 5 dB smaller than a direct path signal as explained in SIWIAK “UWB Channel Model for under 1 GHz (VHF and UHF)”, IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs), IEEE document P802.15-02/505r4, September 2004. Second order reflections, on the other hand, namely those involving two bounces, may already be suppressed by as much as 10 dB. With circular polarization, the strongest multipath reflections are suppressed because of the polarization inversion.
What is needed is a UWB transmission system and methodology that addresses the problems above.